You can potentially use the pen-up/pen-down events in the EggBot control scripts to turn a laser on and off through the EBB.
What I’d suggest is changing the definition of the pen-up and the pen-down commands, to turn the laser off and on, respectively.
The EBB command set documentation is here: http://evil-mad.github.io/EggBot/ebb.html
One of the “fine print” things there is that there a number of “UBW” commands supported, which are documented here:
You’ll want to use the “PO” command from that set, for example “PO,B,0,0” to take pin B0 low, or “PO,B,0,1” to take pin B0 high (I have not tested these commands while writing this — just reading from the docs). Pins B0 and B2 would be particularly good choices, since they are not otherwise used on the EggBot/WaterColorBot/AxiDraw software.